The present invention relates to a reformation reactor with a reaction zone in which a reformation catalyst is located and to which a gas mixture containing a hydrocarbon to be reformed can be supplied.
An area of application for reformation reactors that is becoming increasingly significant is motor vehicles operated by fuel cells. Reformation reactors of this type are used to obtain hydrogen from methanol supplied in liquid form in order to obtain the hydrogen required by the fuel cells. Depending on the processing conditions, the methanol can be converted by endothermal steam reformation, exothermal partial oxidation, or, for example, as an autothermal process by a combination of both reactions, into a reformate gas that is rich in hydrogen. For the sake of simplicity, in the present case the term xe2x80x9creformationxe2x80x9d will also include the partial oxidation of methanol or another hydrocarbon that is used.
The liquid components that participate in the reformation reaction are evaporated before they enter the reaction zone. Usually this takes place either in a separate evaporator connected upstream of the reactor or in a heated reactor area spatially separated from, and connected upstream of, the reaction zone. The gas mixture components can be added to this evaporator area by ultrasonic atomization, for example. Another conventional method consists in the complete combustion of a liquid hydrocarbon and evaporation of the fuel or water added later by the hot combustion products. The problems with this conventional method consist primarily of the inexact monitoring of the evaporation process, which can lead to incomplete evaporation, in other words the formation of drops, pulsations, a high pressure drop, and high heat losses in the case of indirect heating as well as sluggish dynamics. These defects are not advantageous, for example, in the area of application to motor vehicles that operate by fuel cells. For reasons related to space requirements and because all the load changes typical of motor vehicle operation are rapid and frequent, a reactor type is desirable that has a compact design and reacts rapidly to load changes.
U.S. Pat. No. 3,798,005 discloses a reactor for producing a hydrogen-rich gas by catalytic oxidation, preferably of long-chain hydrocarbons such as C6H14 and C8H18. The reactor consists of at least two sintered blocks located in series with a space between them, to which blocks a suitable catalyst material is added. At least the sintered block containing the catalyst and located first in the flow direction is designed for catalytic oxidation of the hydrocarbon, while at least the last sintered block in the flow direction is designed as a shift stage for converting carbon monoxide into carbon dioxide. Catalytic oxidation of the hydrocarbon typically takes place at relatively high temperatures between 900xc2x0 C. and 1650xc2x0 C. while the COxe2x80x94CO2 conversion reaction takes place at temperatures between approximately 150xc2x0 C. and 5000xc2x0 C. The water required for the COxe2x80x94CO2 conversion reaction is injected as steam into the space in front of each shift stage sintered block. In front of the first sintered block in the flow direction that contains the catalyst, there is a mixing chamber into which the hydrocarbon to be reacted is added through an injection line and an air stream is added through a catalyst-free sintered block on the inlet side. The quantity of oxygen in the air stream is adjusted so that the subsequent catalytic oxidation takes place as incomplete flame-free combustion. The air stream is supplied in a countercurrent on the outside of a reactor jacket and is preheated by the heat given off. A pipe complex located on the outlet side in the reactor and consequently subjected to the flow of reaction gas that is still hot is used to preheat the hydrocarbon to be reacted and the water that is injected.
Offenlegungsschrift DE 33 45 958 A1 discloses a methanol reformation reactor for producing hydrogen in a fuel-cell-operated motor vehicle and an operating method therefor in which a special starting phase is provided before subsequent continuous operation. In this starting phase, liquid methanol is burned in a combustion chamber of the reactor that is separate from a reformation reaction zone. The hot combustion gas flows through a combustion gas chamber that is in a heat-conducting connection through a heat-conducting wall with the reformation reaction zone and as a result indirectly heats the catalyst material located therein. In addition, the combustion gas, after passing through the combustion gas chamber, is recycled through the reformation reaction zone, thus also heating the catalyst directly. When the reaction zone has reached its operating temperature, in this manner, methanol combustion is terminated and a hot-gas valve is switched to remove the combustion gas from the system, while during continuous operation that then begins, a methanol/steam mixture is added to the reaction zone for steam reformation of the methanol. A burner that serves to maintain the heating of the reaction zone during continuous operation of the reactor is operated on the surplus hydrogen from the fuel cells.
A reformation reactor with a plate design is disclosed in Offenlegungsschrift JP 5-337358 (A) in which a distribution chamber is laminated to a combustion chamber with interposition of a fuel distributing plate. The distributing plate is provided with a large number of distributing openings. A fuel/air mixture is burned in the combustion chamber. The air is fed directly into the combustion chamber through a matching air feed line, while the fuel enters the distribution chamber through a matching fuel supply opening and travels from there through the openings in the distributing plate into the combustion chamber. In order to prevent backflow from the combustion chamber into the distributing chamber, a portion of the airflow can be supplied to the fuel feed line through a branch line with a controllable valve and premixed therein with the supplied fuel.
Patent DE 37 29 114 C2 discloses a catalytic oxidation reactor for ignitable gas mixtures, in which a gas-permeable first layer containing a suitable oxidation catalyst is contained in a reaction chamber connected with a cooling medium and provided on a side facing the supplied gas mixture with a gas-permeable second layer. On its side opposite the second layer, the first layer is covered by a third layer that is impermeable to gas and liquid and is in thermal contact with the cooling medium. The gas mixture to be reacted passes through the second layer into the first layer where the oxidation reaction takes place under the influence of the catalyst. The resultant reaction components, for example steam in the case of an oxygen/hydrogen conversion, are then brought out from the first layer in a countercurrent through the second layer and then discharged. The second, preferably porous, layer functions as a diffusion blocking layer that allows only a metered predetermined gas mixture quantity to reach the first layer and is composed of a material that is a poor conductor of heat so that heat removal takes place primarily through the third layer to the cooling medium.
The technical problem solved by the present invention is the provision of a reformation reactor and an operating method therefor for reforming a hydrocarbon-containing gas mixture that is especially suited for applications in which a compact reactor design and high reactor dynamics are required, especially for mobile applications, such as fuel-cell-operated motor vehicles. For the sake of simplicity, the term xe2x80x9chydrocarbonxe2x80x9d will be used to refer to hydrocarbon derivatives as well, such as methanol.
The present invention achieves this goal by providing a reformation reactor comprising (1) a reaction zone in which a reformation catalyst is located and to which a hydrocarbon-containing gas mixture to be reformed can be supplied; and (2) an evaporator body that is flush and adjacent to reaction zone, wherein the evaporator body has a porous, heat-conducting structure for preparing the gas mixture by mixing and evaporating gas mixture components supplied in liquid form, and for two-dimensionally feeding the prepared gas mixture into the reaction zone through a corresponding evaporator body and reaction zone interface. The present invention also achieves this goal by providing an operating method comprising adding, in a starting phase, an oxidizable material containing a hydrocarbon as a liquid film to an evaporator body to wet its porous structure; adding a gas containing oxygen; catalytically oxidizing a part of the oxidizable material that makes the transition to the gas phase in the reaction zone, thereby releasing heat that is conducted into the evaporator body; and after an operating temperature of the evaporator body is reached, during a starting phase, supplying components to the evaporator body to prepare the gas mixture to be reformed; and feeding the gas mixture into the reaction zone.
The reactor according to the present invention includes an evaporator body that abuts the reaction zone two-dimensionally. The evaporator body is designed with a porous heat-conducting structure such that it is able to mix and evaporate the components of the gas mixture to be reformed that are supplied to it and to supply them as a homogeneous evaporated mixture distributed two-dimensionally via the interface into the reaction zone. As a result of this integration of the evaporator into the reactor, an especially compact reactor design is possible. As a result of the heat-conducting design of the evaporator body structure, in cases in which an exothermal reaction is taking place in an adjoining area of the reaction zone, surplus heat can be added very effectively and uniformly transported to the evaporator body from the reaction zone by solid-body conduction, and can be used therein for the evaporation process. This heat transport through solid-body conduction is much more effective than heat transport via the gas phase. Since the evaporator body directly abuts the reformation reaction zone and the gas mixture to be reformed is fed homogeneously and two-dimensionally into the reaction zone, very short heat transport pathways result, together with low heat capacities and good spatial homogeneity of the reaction products, so that the reactor has comparatively high dynamics and is thus able to react very rapidly to load changes.
In another embodiment of the present invention, the reaction zone is formed by a reaction body having a porous, heat-conducting structure that is connected with the evaporator body in such a way that the solid bodies conduct heat, and incorporates the reformation catalyst as a surface layer. This surface layer can be much thinner by comparison with the typical diameters of the pellets in conventional catalyst charges. Thus, there is no overheating of the catalyst material and the entire catalyst mass can be used for the reformation reaction, since the gas mixture to be reformed can penetrate by diffusion into the entire, relatively limited depth of the layer. Advantageously, the reaction body can form an integral, porous, heat-conducting structure with the evaporator body, in which only the part that forms the reaction body is provided with the catalyst layer.
In another embodiment of the reactor according to the present invention, a special guide plate is provided by which the components of the gas mixture to be reformed can be supplied, relatively uniformly distributed, to an inlet area of the evaporator body, reinforcing the mixing function of the evaporator body.
In the operating method according to the present invention, a special starting phase is provided for the reformation reactor, in which an oxidizable, hydrocarbon-containing material is added to the evaporator body as a liquid film that wets the porous structure and a gas containing oxygen is added in such fashion that the part of the hydrocarbon-containing material that changes to the gas phase travels from the evaporator body into the reaction zone and is burned catalytically there, with oxidation taking place two-dimensionally on the basis of the special reactor design, thereby avoiding xe2x80x9chot spotsxe2x80x9d and hence triggering of a homogeneous combustion that is no longer flame-free. If necessary, this combustion process can also be continued during the continuous operation that follows. The combustion heat produced in the reaction zone is transported very effectively by solid-body conduction into the evaporator body and results in accelerated evaporation of the material containing the hydrocarbon added there in liquid form. As soon as the evaporator body has reached a certain temperature at which the components of the gas mixture to be reformed can evaporate, the components are added to the evaporator body in such fashion that their complete evaporation is ensured and a monitored temperature increase as well as a uniform temperature distribution take place. This is achieved preferably by a monitored increase in the admixture of the gas mixture to be reformed and/or by a controlled addition of water and/or a suitable control of the fuel/air ratio for the catalytic combustion. During subsequent continuous operation, the liquid and gaseous components of the gas mixture to be reformed, for example, liquid or already completely or partially gaseous methanol and water as well as possibly a gas containing oxygen, such as air, are introduced into the evaporator body.
In another embodiment of the operating method according to the present invention, the reaction conditions during continuous operation of the reactor are adjusted so that in the reaction zone, at least in an area adjoining the evaporator body, an exothermal reaction takes place, for example a partial oxidation of methanol. The reaction heat can be added very effectively to the evaporator body by solid-body heat conduction, and provides the necessary heat to prepare the gas mixture to be reformed, especially for evaporating and/or overheating the mixture components involved which may be liquid. Maintaining these reaction conditions ensures optimal evaporation under all operating states and therefore also a very good dynamic behavior during load changes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.